Add initial support for struct assignment pattern

Structs can be initialized by an assignment pattern. E.g.

```
struct packed {
  int x;
  shortint y;
} S = '{ 1, 2};
```

is the same as

```
struct packed {
  int x;
  shortint y;
} S;
s.x = 1;
s.y = 2;
```

Add initial support for unnamed struct assignment patterns. Named struct
assignment patterns like

```
struct packed {
  int x;
  shortint y;
} S = '{x: 1, y: 2};
```

are still unsupported.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

PExpr.h

@@ -219,6 +219,9 @@ class PEAssignPattern : public PExpr {
 				      const std::vector<netrange_t> &dims,
 				      unsigned int cur_dim,
 				      bool need_const) const;
+      NetExpr* elaborate_expr_struct_(Design *des, NetScope *scope,
+				      const netstruct_t *struct_type,
+				      bool need_const) const;
       NetExpr* elaborate_expr_darray_(Design *des, NetScope *scope,
 				      const netdarray_t *array_type,
 				      bool need_const) const;

elab_expr.cc

@@ -252,6 +252,11 @@ NetExpr*PEAssignPattern::elaborate_expr(Design*des, NetScope*scope,
 					  need_const);
       }
 
+      if (auto struct_type = dynamic_cast<const netstruct_t*>(ntype)) {
+	    return elaborate_expr_struct_(des, scope, struct_type,
+					  need_const);
+      }
+
       cerr << get_fileline() << ": sorry: I don't know how to elaborate "
 	   << "assignment_pattern expressions for " << *ntype << " type yet." << endl;
       cerr << get_fileline() << ":      : Expression is: " << *this
@@ -336,6 +341,33 @@ NetExpr* PEAssignPattern::elaborate_expr_packed_(Design *des, NetScope *scope,
       return concat;
 }
 
+NetExpr* PEAssignPattern::elaborate_expr_struct_(Design *des, NetScope *scope,
+						 const netstruct_t *struct_type,
+						 bool need_const) const
+{
+      auto &members = struct_type->members();
+
+      if (members.size() != parms_.size()) {
+	    cerr << get_fileline() << ": error: Struct assignment pattern expects "
+	         << members.size() << " element(s) in this context.\n"
+	         << get_fileline() << ":      : Found "
+		 << parms_.size() << " element(s)." << endl;
+	    des->errors++;
+      }
+
+      NetEConcat *concat = new NetEConcat(parms_.size(), 1,
+					  struct_type->base_type());
+      for (size_t idx = 0; idx < std::min(parms_.size(), members.size()); idx++) {
+	    auto expr = elaborate_rval_expr(des, scope,
+					    members[idx].net_type,
+					    parms_[idx], need_const);
+	    if (expr)
+		  concat->set(idx, expr);
+      }
+
+      return concat;
+}
+
 NetExpr* PEAssignPattern::elaborate_expr(Design*des, NetScope*, unsigned, unsigned) const
 {
       cerr << get_fileline() << ": sorry: I do not know how to"

netstruct.h

@@ -65,6 +65,7 @@ class netstruct_t : public LineInfo, public ivl_type_s {
 	// description, and set the off value to be the offset into
 	// the packed value where the member begins.
       const struct member_t* packed_member(perm_string name, unsigned long&off) const;
+      const std::vector<member_t>& members() const { return members_; }
 
 	// Return the width (in bits) of the packed record, or -1 if
 	// the record is not packed.